1. Field of the Invention
The present invention relates to a video telephone terminal connected to an Asynchronous Transfer Mode (ATM) network, and in particular, to a video telephone terminal interworking with an Integrated Services Digital Network (ISDN).
2. Description of the Related Art
Video telephones have been provided according to the H.320 terminal standard as ISDN offered a new telephone service. Over the past few years, video telephone services have been offered on the existing POTS (Plain Old Telephone System) line using a modulator-demodulator (MODEM) and on a local area network (LAN), respectively. The H.324 and H.323 terminal standards are recommended respectively for the video telephone services on the POTS line using a MODEM and on LAN. For interworking of these video telephone services with broadband-ISDN (B-ISDN), the H.321 terminal standard is recommended.
Since the 1980's, many solutions have been proposed to provide video telephone service on ISDN but they have failed to come into wide use. However, the recent development in media processor technology enables implementation of an H.261 video CODEC on a single integrated circuit (IC) chip. Besides, the video CODEC, a G.728 audio CODEC, and a multiplexer/demultiplexer can be integrated on a single chip in some components. Video telephony is effected through an analog MODEM over a telephone network, and can be implemented on a personal computer (PC) simply with software. Microsoft Corporation provides a software called Netmeeting which satisfies the H.323 standard for video teleconference over a LAN. Despite unsatisfactory image quality, which is attributed to the transmission speed limits resulting from the present video telephone service with Point-to-Point Protocol (PPP) through a MODEM over a telephone network, realization of a highly advanced network, that is, an ultra high-speed information communication network will lead to wider provision of telephone service in the future.
When video telephones are widely provided from this perspective, it is expected that a video telephone platform on a circuit switching network such as ISDN coexists with a video telephone platform on LAN. Therefore, an ultra high-speed communication network, such as an ATM network, should accommodate the different service platforms of these two networks, and they should be easily converted or selected by a terminal. A description of the video telephone platforms on the circuit switching network and on LAN follow H.320 and H.323, respectively.
The International Telecommunication Union (ITU) recommends H.321 as an ATM video telephone terminal standard interworking with the video telephone platform on ISDN. H.321 is characterized by additional techniques of emulating a circuit switching network such as ISDN on an ATM network. LAN emulation and Internet Protocol (IP) over ATM have been provided as ways to accommodate the existing LAN network on the ATM network, and a standard for Multi-Protocol Over ATM (MPOA) evolved from them has recently been suggested. Since these techniques are performed under a network layer, an H.323 video telephone platform can easily be realized on the ATM network by modifying a user-to-network interface in a network connecting portion without a new terminal standard. Replacement of a network interface card (NIC) and a device driver provides an environment like the existing LAN.
However, in order to realize H.321, a circuit switching network should be emulated in a user-to-user protocol as well as the user-to-network interface protocol. To prevent data loss on a data buffer in a receiving end or a relay, clock pulses of audio and video sources should be recovered in emulating the circuit switching network for the user data. For this circuit switching network emulation, the ITU recommends an ATM Adaptation Layer (AAL) type 1, that is, AAL-1.
However, difficulties are involved in actual implementation of H.321 because a signalling protocol for a user-to-network interface and almost every service on ATM uses AAL-5, and thus both AAL-5 and AAL-1 should be accommodated. Since simultaneous accommodation of AAL-5 and AAL-1 increases the cost of a terminal in view of system architecture and they require different statistical characteristics and specifications of data, there is no chip solution for accommodating them on a single chip. In AAL-5, a packet data is simply mapped into an ATM cell, whereas in AAL-1, a source clock frequency should be recovered by a phase locked loop (PLL) to map real time bit stream data such as voice and images into an ATM cell.
As described above, because simultaneous accommodation of AAL-5 and AAL-1 results in the increase of terminal cost in view of system architecture, and statistical characteristics and specification requirements of data are different, there is no chip solution to accommodate them on a single chip.